Ecosystem Monitoring & Assessment (EMA) Program

BASIS: Linking Recent Changes in Ocean Conditions With Pelagic Fish Recruitment

Fig. 1, click image to enlarge. Fig. 2, click image to enlarge. Fig. 3, click image to enlarge.

The AFSC conducts regular, comprehensive trawl surveys of marine fish and invertebrate populations in the Bering Sea using chartered commercial fishing vessels. The established survey design is to conduct annual surveys of continental slope groundfish in the Bering Sea. Auke Bay Laboratories (ABL) Ecosystem Monitoring and Assessment (EMA) program conducts these comprehensive trawl surveys and related research in the exclusive economic zone off Alaska as required to monitor and describe trends in abundance and distribution of commercially and ecologically valuable groundfish populations.

Scientists within the EMA program conduct these comprehensive surveys as part of the Bering Aleutian Salmon International Survey (BASIS) designed to improve our collective understanding of fisheries and oceanographic ecosystem indices in the North Pacific Ocean and to clarify mechanisms linking recent changes in ocean conditions with pelagic fish recruitment.

Climate models predict a gradual increase in atmospher­ic temperature, with the greatest increases occurring in sub-arctic and arctic regions. The evidence for current warming trends is the poleward retreat of seasonal sea ice cover in the Arctic. Continued warming is predicted to have a profound effect on Bering Sea ecosystems. For instance, there is evidence that climate warming will increase water column stability on the eastern Bering Sea shelf, limiting the flux of nutrients into the photic zone and perhaps negatively impacting primary and secondary productivity. In addition, large-scale climate cycles are affecting regional climate trends.

Shifts in the position of the Far Eastern Low and Aleutian Low pressure systems determine whether or not the Bering Sea experiences warming or cooling and also affects the velocity of ocean currents. The position of these atmospheric low pressure systems (NE and W, respec­tively) during 2002 to 2005 brought warmer air to the Ber­ing Sea during winter and was related to decreased storm activity during summer. The position of these low pressure systems shifted again (SW and E, respectively) during 2006, resulting in colder arctic air covering much of the Bering Sea during winter and summer which increased storm activity.

Identification of linkages between climate and the ecosystem requires a conceptual framework in which to compare observational data and build numerical models. BASIS collects observational data on the biological characteristics of fish and subsequent physical and biological oceanographic data that may affect fish recruitment. The surveys began in 2002, a time of anomalously warm spring and summer sea tempera­tures. These warm sea temperatures continued through 2005, switching to anomalously cold from 2006 to 2010.

The survey is conducted during late summer to early fall to address the Critical Size and Period hypothesis for juvenile (Pacific salmon) and age-0 pelagic fish (walleye pollock and Pacific cod) species. Fish that do not reach a critical size and energetic fitness prior to their first winter at sea may sustain higher overwinter mortality.

BASIS data have been used to support the predictions that, in years with early ice retreat, more primary production remains in the pelagic environment of the eastern Bering Sea, and that high numbers of age-0 pollock survive to summer. However, in these years, production of large crustacean zooplankton is reduced (Fig. 1), depriving age-0 pollock of lipid-rich prey in summer and fall. Consequently, age-0 pollock energy reserves (Fig. 2) are low and predation on them is increased. The result is weak recruitment of age-1 recruits the following year (Fig. 3). A revised oscillating control hypothesis indicates bottom-up constraints on pollock recruitment in very warm periods. Prolonged warm periods with decreased ice cover will likely cause diminished pollock recruitment and catches relative to recent values.

For more information on the effects of climate on marine ecosystems of the eastern Bering Sea, see Coyle et al. (2011) and Hunt et al. (in press).

By Ed Farley